Abstract. Periodic wave structures in the thermosphere have been observed at São João do Cariri (geographic coordinates: 36.5° W, 7.4° S; geomagnetic coordinates based on IGRF model to 2015: 35.8° E, 0.48° N) from September 2000 to November 2010 using OI630.0 nm airglow images. During this period, which corresponds to almost one solar cycle, characteristics of 98 waves were studied. Similarities between the characteristics of these events and observations at other places around the world were noted, primarily the spectral parameters. The observed periods were mostly found between 10 and 35 min; horizontal wavelengths ranged from 100 to 200 km, and phase speed from 30 to 180 m s−1. These parameters indicated that some of the waves, presented here, are slightly faster than those observed previously at low and middle latitudes (Indonesia, Carib and Japan), indicating that the characteristics of these waves may change at different places. Most of observed waves have appeared during magnetically quiet nights, and the occurrence of those waves followed the solar activity. Another important characteristic is the quasi-monochromatic periodicity that distinguish them from the single-front medium-scale traveling ionospheric disturbances (MSTIDs) that have been observed previously over the Brazilian region. Moreover, most of the observed waves did not present a phase front parallel to the northeast–southwest direction, which is predicted by the Perkins instability process. It strongly suggests that most of these waves must have had different generation mechanisms from the Perkins instability, which have been pointed out as being a very important mechanism for the generation of MSTIDs in the lower thermosphere.
a b s t r a c tEquatorial ionosphere plasma bubbles over the South American continent were successfully observed by mapping the total electron content (TECMAP) using data provided by ground-based GNSS receiver networks. The TECMAP could cover almost all of the continent within $ 4000 km distance in longitude and latitude, monitoring TEC variability continuously with a time resolution of 10 min. Simultaneous observations of OI 630 nm all-sky image at Cachoeira Paulista (22.7°S, 45.0°W) and Cariri (7.4°S, 36.5°W) were used to compare the bubble structures. The spatial resolution of the TECMAP varied from 50 km to 1000 km, depending on the density of the observation sites. On the other hand, optical imaging has a spatial resolution better than 15 km, depicting the fine structure of the bubbles but covering a limited area ( $ 1600 km diameter). TECMAP has an advantage in its spatial coverage and the continuous monitoring (day and night) form. The initial phase of plasma depletion in the post-sunset equatorial ionization anomaly (PS-EIA) trough region, followed by development of plasma bubbles in the crest region, could be monitored in a progressive way over the magnetic equator. In December 2013 to January 2014, periodically spaced bubble structures were frequently observed. The longitudinal spacing between the bubbles was around 600-800 km depending on the day. The periodic form of plasma bubbles may suggest a seeding process related to the solar terminator passage in the ionosphere.
Abstract. Periodic waves were observed in the OI6300 airglow images over São
João do Cariri (36.5∘ W, 7.4∘ S) from 2012 to
2014 with simultaneous observations of the thermospheric wind using
two Fabry–Pérot interferometers (FPIs). The FPIs measurements were carried out
at São João do Cariri and Cajazeiras (38.5∘ W,
6.9∘ S). The observed spectral characteristics of these
waves (period and wavelength) as well the propagation direction were estimated
using two-dimensional Fourier analysis in the
airglow images. The horizontal thermospheric wind was calculated
from the Doppler shift of the OI6300 data extracted from interference
fringes registered by the FPIs. Combining these two techniques, the
intrinsic parameters of the periodic waves were estimated and
analyzed. The spectral parameters of the periodic waves were quite
similar to the previous observations at São João do Cariri. The
intrinsic periods for most of the waves were shorter than the
observed periods, as a consequence, the intrinsic phase speeds were
faster compared to the observed phase speeds. As a consequence, these
waves can easily propagate into the thermosphere–ionosphere since
the fast gravity waves can skip turning and critical levels. The
strength and direction of the wind vector in the thermosphere must be
the main cause for the observed anisotropy in the propagation
direction of the periodic waves, even if the sources of these waves
are assumed to be isotropic. Keywords. Meteorology and atmospheric dynamics (waves and tides)
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